The present invention relates to a sheet for conducting heat. The heat conductive sheet of the invention can be used in electronic appliances, for example, by being interposed between a heat generating electronic component and a radiator, or between an electronic component and a metallic heat exchanger plate.
With recent progress in performance of the electronic appliances, such as central processor unit (CPU) of computers, the power consumption is also increasing. The power consumption increases heat generation and the generated heat has an adverse effect on operation of the computers. Therefore, a technique for efficiently cooling the heat generated at the electronic devices is very important for maintaining the performance of the computers.
The heat conductive sheet of the invention is used in electronic appliances, for example, by being interposed between a heat generating electronic component and a radiator, or between an electronic component and a metallic heat exchanger plate. The heat conductive sheet which conducts heat from the electronic component to the radiator must have high thermal conductivity in order to efficiently cool the electronic devices as described above.
Japanese Laid-Open Patent Publication 63-20340 discloses a heat conductive sheet having a high thermal conductivity by densely filling the sheet with a heat conductive filler. The heat conductive filler comprises spherical alumina grains mixed into a composition comprising a synthetic resin and a rubber having high thermal conductivity. The spherical alumina grains have a shape devoid of cutting edges, have a maximum grain diameter of 150 xcexcm, and have an average grain diameter between 5 and 35 xcexcm. The spherical alumina grains described above can be easily dispersed into organic polymers and can be densely filled so that the spherical alumina grains are extremely useful for heat conductive filler used in a heat conductive sheet.
In addition to Japanese Laid-Open Patent Publication 63-20340, Japanese Laid-Open Patent Publications 11-87958 and 2000-1616 disclose the use of alumina fillers. Japanese Laid-Open Patent Publication 11-87958 discloses a filler in which alumina filler and another heat conductive filler are combined. Japanese Laid-Open Patent Publication 2000-1616 is directed to an organic matrix for mixing alumina at a high filling factor.
However, while attempts to densely fill the heat conductive filler have been made, the mechanism of how the high heat conductivity is attained by the high filling factor of the heat conductive filler has not been examined sufficiently. Therefore in some cases, the material including filler at a high filling factor due to dense filling of spherical alumina could have a lower thermal conductivity than that expected from the filler amount.
The present invention has been made in recognition of the problems that existed in conventional art. An object of the invention is to provide a heat conductive sheet that sufficiently realizes the thermal conductivity that complies with high filler amount of the heat conductive filler.
In order to achieve the above objects, the present invention provides a heat conductive sheet comprising organic matrix and heat conductive filler contained in the organic matrix at a volume percent between 30% and 80%, wherein the heat conductive filler comprises spherical shaped alumina grains that have an average grain diameter in a range between 50 xcexcm and 80 xcexcm and that include micro-grains having a grain diameter of 30 xcexcm or less at a proportion of 5 volume % or less with respect to the total amount of the spherical shaped alumina grains.
Other aspects and advantages of the invention will become apparent from the following description illustrating by way of example the principles of the invention.